1.
Constellation
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A constellation is formally defined as a region of the celestial sphere, with boundaries laid down by the International Astronomical Union. The constellation areas mostly had their origins in Western-traditional patterns of stars from which the constellations take their names, in 1922, the International Astronomical Union officially recognized the 88 modern constellations, which cover the entire sky. They began as the 48 classical Greek constellations laid down by Ptolemy in the Almagest, Constellations in the far southern sky are late 16th- and mid 18th-century constructions. 12 of the 88 constellations compose the zodiac signs, though the positions of the constellations only loosely match the dates assigned to them in astrology. The term constellation can refer to the stars within the boundaries of that constellation. Notable groupings of stars that do not form a constellation are called asterisms, when astronomers say something is “in” a given constellation they mean it is within those official boundaries. Any given point in a coordinate system can unambiguously be assigned to a single constellation. Many astronomical naming systems give the constellation in which an object is found along with a designation in order to convey a rough idea in which part of the sky it is located. For example, the Flamsteed designation for bright stars consists of a number, the word constellation seems to come from the Late Latin term cōnstellātiō, which can be translated as set of stars, and came into use in English during the 14th century. It also denotes 88 named groups of stars in the shape of stellar-patterns, the Ancient Greek word for constellation was ἄστρον. Colloquial usage does not draw a distinction between constellation in the sense of an asterism and constellation in the sense of an area surrounding an asterism. The modern system of constellations used in astronomy employs the latter concept, the term circumpolar constellation is used for any constellation that, from a particular latitude on Earth, never sets below the horizon. From the North Pole or South Pole, all constellations south or north of the equator are circumpolar constellations. In the equatorial or temperate latitudes, the term equatorial constellation has sometimes been used for constellations that lie to the opposite the circumpolar constellations. They generally include all constellations that intersect the celestial equator or part of the zodiac, usually the only thing the stars in a constellation have in common is that they appear near each other in the sky when viewed from the Earth. In galactic space, the stars of a constellation usually lie at a variety of distances, since stars also travel on their own orbits through the Milky Way, the star patterns of the constellations change slowly over time. After tens to hundreds of thousands of years, their familiar outlines will become unrecognisable, the terms chosen for the constellation themselves, together with the appearance of a constellation, may reveal where and when its constellation makers lived. The earliest direct evidence for the constellations comes from inscribed stones and it seems that the bulk of the Mesopotamian constellations were created within a relatively short interval from around 1300 to 1000 BC

2.
Canis Major
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Canis Major /ˌkeɪnᵻs ˈmeɪdʒər/ is a constellation in the southern celestial hemisphere. In the second century, it was included in Ptolemys 48 constellations and its name is Latin for greater dog in contrast to Canis Minor, the lesser dog, both figures are commonly represented as following the constellation of Orion the hunter through the sky. The Milky Way passes through Canis Major and several open clusters lie within its borders, Canis Major contains Sirius, the brightest star in the night sky, known as the dog star. It is bright because of its proximity to the Solar System, in contrast, the other bright stars of the constellation are stars of great distance and high luminosity. At magnitude 1.5, Epsilon Canis Majoris is the second-brightest star of the constellation, the red hypergiant VY Canis Majoris is one of the largest stars known, while the neutron star RX J0720. 4-3125 has a radius of a mere 5 km. In ancient Mesopotamia, Sirius, named KAK. SI, in the later compendium of Babylonian astronomy and astrology titled MUL. APIN, the arrow, Sirius, was also linked with the warrior Ninurta, and the bow with Ishtar, daughter of Enlil. Ninurta was linked to the later deity Marduk, who was said to have slain the ocean goddess Tiamat with a great bow, the Ancient Greeks replaced the bow and arrow depiction with that of a dog. It was also considered to represent one of Orions hunting dogs, pursuing Lepus the Hare or helping Orion fight Taurus the Bull, the ancient Greeks refer only to one dog, but by Roman times, Canis Minor appears as Orions second dog. Alternative names include Canis Sequens and Canis Alter, Canis Syrius was the name used in the 1521 Alfonsine tables. The Roman myth refers to Canis Major as Custos Europae, the dog guarding Europa but failing to prevent her abduction by Jupiter in the form of a bull, and as Janitor Lethaeus, the watchdog. In medieval Arab astronomy, the constellation became Al Kalb al Akbar, islamic scholar Abū Rayḥān al-Bīrūnī referred to Orion as Al Kalb al Jabbār, the Dog of the Giant. Among the Merazig of Tunisia, shepherds note six constellations that mark the passage of the dry, one of them, called Merzem, includes the stars of Canis Major and Canis Minor and is the herald of two weeks of hot weather. In Chinese astronomy, the constellation of Canis Major lies in the Vermilion Bird. The Military Market was a pattern of stars containing Nu3, Beta, Xi1 and Xi2. The Wild Cockerel was at the centre of the Military Market, schlegel reported that the stars Omicron and Pi Canis Majoris might have been them, while Beta or Nu2 have also been proposed. Sirius was Tiānláng, the Celestial Wolf, denoting invasion and plunder, southeast of the Wolf was the asterism Húshǐ, the celestial Bow and Arrow, which was interpreted as containing Delta, Epsilon, Eta and Kappa Canis Majoris and Delta Velorum. Alternatively, the arrow was depicted by Omicron2 and Eta and aiming at Sirius, while the bow comprised Kappa, Epsilon, Sigma, Delta and 164 Canis Majoris, and Pi and Omicron Puppis. Both the Māori people and the people of the Tuamotus recognized the figure of Canis Major as a distinct entity, Te Huinga-o-Rehua, also called Te Putahi-nui-o-Rehua and Te Kahui-Takurua, was a Maori constellation that included both Canis Minor and Canis Major, along with some surrounding stars

3.
Cosmic distance ladder
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The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an object is possible only for those objects that are close enough to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at distances and methods that work at larger distances. Several methods rely on a candle, which is an astronomical object that has a known luminosity. The ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy, instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next higher rung, at the base of the ladder are fundamental distance measurements, in which distances are determined directly, with no physical assumptions about the nature of the object in question. The precise measurement of stellar positions is part of the discipline of astrometry, direct distance measurements are based upon the astronomical unit, which is the distance between the Earth and the Sun. Historically, observations of transits of Venus were crucial in determining the AU, in the first half of the 20th century, observations of asteroids were also important. Keplers laws provide precise ratios of the sizes of the orbits of objects orbiting the Sun, radar is used to measure the distance between the orbits of the Earth and of a second body. From that measurement and the ratio of the two sizes, the size of Earths orbit is calculated. The Earths orbit is known with a precision of a few meters, the most important fundamental distance measurements come from trigonometric parallax. As the Earth orbits the Sun, the position of stars will appear to shift slightly against the more distant background. These shifts are angles in a triangle, with 2 AU making the base leg of the triangle. The amount of shift is small, measuring 1 arcsecond for an object at the 1 parsec distance of the nearest stars. Astronomers usually express distances in units of parsecs, light-years are used in popular media, because parallax becomes smaller for a greater stellar distance, useful distances can be measured only for stars whose parallax is larger than a few times the precision of the measurement. Parallax measurements typically have an accuracy measured in milliarcseconds, the Hubble telescope WFC3 now has the potential to provide a precision of 20 to 40 microarcseconds, enabling reliable distance measurements up to 5,000 parsecs for small numbers of stars. By the early 2020s, the GAIA space mission will provide similarly accurate distances to all bright stars. Stars have a velocity relative to the Sun that causes proper motion, for a group of stars with the same spectral class and a similar magnitude range, a mean parallax can be derived from statistical analysis of the proper motions relative to their radial velocities

4.
Star
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A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun, many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the stars in the Universe, including all stars outside our galaxy, indeed, most are invisible from Earth even through the most powerful telescopes. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the stars lifetime, near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity, and many properties of a star by observing its motion through space, its luminosity. The total mass of a star is the factor that determines its evolution. Other characteristics of a star, including diameter and temperature, change over its life, while the environment affects its rotation. A plot of the temperature of stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that allows the age and evolutionary state of that star to be determined. A stars life begins with the collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, the remainder of the stars interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The stars internal pressure prevents it from collapsing further under its own gravity, a star with mass greater than 0.4 times the Suns will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements at the core or in shells around the core, as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars. Meanwhile, the core becomes a remnant, a white dwarf. Binary and multi-star systems consist of two or more stars that are bound and generally move around each other in stable orbits. When two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, historically, stars have been important to civilizations throughout the world

5.
Earth
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Earth, otherwise known as the World, or the Globe, is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets, according to radiometric dating and other sources of evidence, Earth formed about 4.54 billion years ago. Earths gravity interacts with objects in space, especially the Sun. During one orbit around the Sun, Earth rotates about its axis over 365 times, thus, Earths axis of rotation is tilted, producing seasonal variations on the planets surface. The gravitational interaction between the Earth and Moon causes ocean tides, stabilizes the Earths orientation on its axis, Earths lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earths surface is covered with water, mostly by its oceans, the remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earths polar regions are covered in ice, including the Antarctic ice sheet, Earths interior remains active with a solid iron inner core, a liquid outer core that generates the Earths magnetic field, and a convecting mantle that drives plate tectonics. Within the first billion years of Earths history, life appeared in the oceans and began to affect the Earths atmosphere and surface, some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earths distance from the Sun, physical properties, in the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely, over 7.4 billion humans live on Earth and depend on its biosphere and minerals for their survival. Humans have developed diverse societies and cultures, politically, the world has about 200 sovereign states, the modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most often spelled eorðe. It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō, originally, earth was written in lowercase, and from early Middle English, its definite sense as the globe was expressed as the earth. By early Modern English, many nouns were capitalized, and the became the Earth. More recently, the name is simply given as Earth. House styles now vary, Oxford spelling recognizes the lowercase form as the most common, another convention capitalizes Earth when appearing as a name but writes it in lowercase when preceded by the. It almost always appears in lowercase in colloquial expressions such as what on earth are you doing, the oldest material found in the Solar System is dated to 4. 5672±0.0006 billion years ago. By 4. 54±0.04 Gya the primordial Earth had formed, the formation and evolution of Solar System bodies occurred along with the Sun

6.
Sirius
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Sirius is a star system and the brightest star in the Earths night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the system has the Bayer designation Alpha Canis Majoris. The distance separating Sirius A from its companion varies between 8.2 and 31.5 AU, Sirius appears bright because of its intrinsic luminosity and its proximity to Earth. At a distance of 2.6 parsecs, as determined by the Hipparcos astrometry satellite, Sirius is gradually moving closer to the Solar System, so it will slightly increase in brightness over the next 60,000 years. After that time its distance will begin to increase and it will become fainter, Sirius A is about twice as massive as the Sun and has an absolute visual magnitude of 1.42. It is 25 times more luminous than the Sun but has a lower luminosity than other bright stars such as Canopus or Rigel. The system is between 200 and 300 million years old and it was originally composed of two bright bluish stars. Sirius is also known colloquially as the Dog Star, reflecting its prominence in its constellation, the brightest star in the night sky, Sirius is recorded in the earliest astronomical records. Every year, it disappears for seventy days before returning to the sky just before sunrise. This occurs at Cairo on 19 July, placing it just prior to the summer solstice, the ancient Greeks observed that the appearance of Sirius heralded the hot and dry summer and feared that it caused plants to wilt, men to weaken, and women to become aroused. Due to its brightness, Sirius would have been noted to twinkle more in the weather conditions of early summer. To Greek observers, this signified certain emanations which caused its malignant influence, anyone suffering its effects was said to be star-struck. It was described as burning or flaming in literature, the season following the stars reappearance came to be known as the dog days. The inhabitants of the island of Ceos in the Aegean Sea would offer sacrifices to Sirius and Zeus to bring cooling breezes, if it rose clear, it would portend good fortune, if it was misty or faint then it foretold pestilence. Coins retrieved from the island from the 3rd century BC feature dogs or stars with emanating rays, ptolemy of Alexandria mapped the stars in Books VII and VIII of his Almagest, in which he used Sirius as the location for the globes central meridian. He depicted it as one of six red-coloured stars, the other five are class M and K stars, such as Arcturus and Betelgeuse. Bright stars were important to the ancient Polynesians for navigation between the islands and atolls of the Pacific Ocean. Low on the horizon, they acted as stellar compasses and they also served as latitude markers, the declination of Sirius matches the latitude of the archipelago of Fiji at 17°S and thus passes directly over the islands each night

7.
Stellar classification
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In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with absorption lines, each line indicates an ion of a certain chemical element, with the line strength indicating the abundance of that ion. The relative abundance of the different ions varies with the temperature of the photosphere, the spectral class of a star is a short code summarizing the ionization state, giving an objective measure of the photospheres temperature and density. Most stars are classified under the Morgan–Keenan system using the letters O, B, A, F, G, K, and M. Each letter class is subdivided using a numeric digit with 0 being hottest and 9 being coolest. The sequence has been expanded with classes for other stars and star-like objects that do not fit in the system, such as class D for white dwarfs. In the MK system, a luminosity class is added to the class using Roman numerals. This is based on the width of absorption lines in the stars spectrum. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a temperature around 5,800 K, the conventional color description takes into account only the peak of the stellar spectrum. This means that the assignment of colors of the spectrum can be misleading. There are no green, indigo, or violet stars, likewise, the brown dwarfs do not literally appear brown. The modern classification system is known as the Morgan–Keenan classification, each star is assigned a spectral class from the older Harvard spectral classification and a luminosity class using Roman numerals as explained below, forming the stars spectral type. The spectral classes O through M, as well as more specialized classes discussed later, are subdivided by Arabic numerals. For example, A0 denotes the hottest stars in the A class, fractional numbers are allowed, for example, the star Mu Normae is classified as O9.7. The Sun is classified as G2, the conventional color descriptions are traditional in astronomy, and represent colors relative to the mean color of an A-class star, which is considered to be white. The apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars. However, most stars in the sky, except the brightest ones, red supergiants are cooler and redder than dwarfs of the same spectral type, and stars with particular spectral features such as carbon stars may be far redder than any black body. O-, B-, and A-type stars are called early type

8.
VY Canis Majoris
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VY Canis Majoris is a red hypergiant star located in the constellation Canis Major. It is one of the largest stars and also one of the most luminous of its type and it has a radius of approximately 1,420 solar radii, and is 17 ±8 times as massive as the Sun. Located about 1.2 kiloparsecs from Earth, it is too faint to be seen with the naked eye, VY Canis Majoris is a single star categorized as a semiregular variable with an estimated period of 2,000 days. It has a density of 5 to 10 mg/m3. If placed at the center of the Solar System, VY Canis Majoriss surface would extend beyond the orbit of Jupiter, although there is still considerable variation in estimates of the radius. The first known recorded observation of VY Canis Majoris is in the catalogue of Jérôme Lalande, on 7 March 1801. Further 19th-century studies of its apparent magnitude demonstrate that the star has been fading since 1850, since 1847, VY Canis Majoris has been described as a crimson star. During the 19th century, observers measured at least six discrete components and these discrete components are now known to be bright areas in the surrounding nebula. Visual observations in 1957 and high-resolution imaging in 1998 showed that there are no companion stars, the varying brightness of VY Cma was not described until 1931 when it was listed as a long period variable with a photographic magnitude range of 9.5 -11.5. It was given the variable star designation VY CMa in 1939, the spectrum of VY CMa is that of a luminous class M star. The hydrogen lines have P Cygni profiles, the spectrum is dominated by TiO bands whose strength suggests a classification of M5. The Hα line is not seen and there are emission lines of neutral sodium and calcium. The luminosity class as determined from different spectral features varies from bright giant to bright supergiant, early attempts at classification were confused by the interpretation of surrounding nebulosity as companion stars. The derived spectral class varies depending on the features examined, the spectral features also vary over time. It is considered to be cooler than M2 and is usually classified between M3 and M4. Classes as extreme as M2.5 and M5 have been given, the luminosity class is likewise confused and often given only as I. During its main sequence, it would have been an O-type star with a mass of 15 to 35 M☉, in 2006, University of Minnesota Professor Roberta M. Humphreys used the spectral energy distribution distance of VY Canis Majoris to calculate its luminosity. 6×105 L☉. A more recent and accurate VLTI measurement gives the star a radius of 1,420 ±120 solar radii, in 1976, Charles J. Lada and Mark J. Reid published the discovery of a bright-rimmed molecular cloud 15 minutes of arc east of VY Canis Majoris

9.
Hipparcos
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Hipparcos was a scientific satellite of the European Space Agency, launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the measurement of the positions of celestial objects on the sky. This permitted the determination of proper motions and parallaxes of stars, allowing a determination of their distance. When combined with radial velocity measurements from spectroscopy, this pinpointed all six quantities needed to determine the motion of stars, the resulting Hipparcos Catalogue, a high-precision catalogue of more than 118,200 stars, was published in 1997. The lower-precision Tycho Catalogue of more than a million stars was published at the same time, Hipparcos follow-up mission, Gaia, was launched in 2013. Problems were dominated by the effects of the Earths atmosphere, but were compounded by complex optical terms, thermal and gravitational instrument flexures, a formal proposal to make these exacting observations from space was first put forward in 1967. Although originally proposed to the French space agency CNES, it was considered too complex and its acceptance within the European Space Agencys scientific programme, in 1980, was the result of a lengthy process of study and lobbying. The spacecraft carried a single all-reflective, eccentric Schmidt telescope, with an aperture of 29 cm, a special beam-combining mirror superimposed two fields of view,58 degrees apart, into the common focal plane. This complex mirror consisted of two mirrors tilted in opposite directions, each occupying half of the entrance pupil. The telescope used a system of grids, at the surface, composed of 2688 alternate opaque and transparent bands. The apparent angle between two stars in the fields of view, modulo the grid period, was obtained from the phase difference of the two star pulse trains. An additional photomultiplier system viewed a beam splitter in the path and was used as a star mapper. Its purpose was to monitor and determine the attitude, and in the process. These measurements were made in two broad bands approximately corresponding to B and V in the UBV photometric system. The positions of these stars were to be determined to a precision of 0.03 arc-sec. The spacecraft spun around its Z-axis at the rate of 11.25 revolutions/day at an angle of 43° to the Sun, the Z-axis rotated about the sun-satellite line at 6.4 revolutions/year. The spacecraft consisted of two platforms and six panels, all made of aluminum honeycomb. The solar array consisted of three sections, generating around 300 W in total

10.
Apparent magnitude
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The apparent magnitude of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value, the Sun, at apparent magnitude of −27, is the brightest object in the sky. It is adjusted to the value it would have in the absence of the atmosphere, furthermore, the magnitude scale is logarithmic, a difference of one in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry, apparent magnitudes are used to quantify the brightness of sources at ultraviolet, visible, and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or often simply as V, the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes. The brightest stars in the sky were said to be of first magnitude, whereas the faintest were of sixth magnitude. Each grade of magnitude was considered twice the brightness of the following grade and this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest, and is generally believed to have originated with Hipparchus. This implies that a star of magnitude m is 2.512 times as bright as a star of magnitude m +1 and this figure, the fifth root of 100, became known as Pogsons Ratio. The zero point of Pogsons scale was defined by assigning Polaris a magnitude of exactly 2. However, with the advent of infrared astronomy it was revealed that Vegas radiation includes an Infrared excess presumably due to a disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures, however, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the scale was extrapolated to all wavelengths on the basis of the black body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, with the modern magnitude systems, brightness over a very wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30, astronomers have developed other photometric zeropoint systems as alternatives to the Vega system. The AB magnitude zeropoint is defined such that an objects AB, the dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of exactly 100. Since an increase of 5 magnitudes corresponds to a decrease in brightness by a factor of exactly 100, each magnitude increase implies a decrease in brightness by the factor 5√100 ≈2.512. Inverting the above formula, a magnitude difference m1 − m2 = Δm implies a brightness factor of F2 F1 =100 Δ m 5 =100.4 Δ m ≈2.512 Δ m

11.
Parsec
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The parsec is a unit of length used to measure large distances to objects outside the Solar System. One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond, a parsec is equal to about 3.26 light-years in length. The nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun, most of the stars visible to the unaided eye in the nighttime sky are within 500 parsecs of the Sun. The parsec unit was likely first suggested in 1913 by the British astronomer Herbert Hall Turner, named from an abbreviation of the parallax of one arcsecond, it was defined so as to make calculations of astronomical distances quick and easy for astronomers from only their raw observational data. Partly for this reason, it is still the unit preferred in astronomy and astrophysics, though the light-year remains prominent in science texts. This corresponds to the definition of the parsec found in many contemporary astronomical references. Derivation, create a triangle with one leg being from the Earth to the Sun. As that point in space away, the angle between the Sun and Earth decreases. A parsec is the length of that leg when the angle between the Sun and Earth is one arc-second. One of the oldest methods used by astronomers to calculate the distance to a star is to record the difference in angle between two measurements of the position of the star in the sky. The first measurement is taken from the Earth on one side of the Sun, and the second is approximately half a year later. The distance between the two positions of the Earth when the two measurements were taken is twice the distance between the Earth and the Sun. The difference in angle between the two measurements is twice the angle, which is formed by lines from the Sun. Then the distance to the star could be calculated using trigonometry. 5-parsec distance of 61 Cygni, the parallax of a star is defined as half of the angular distance that a star appears to move relative to the celestial sphere as Earth orbits the Sun. Equivalently, it is the angle, from that stars perspective. The star, the Sun and the Earth form the corners of a right triangle in space, the right angle is the corner at the Sun. Therefore, given a measurement of the angle, along with the rules of trigonometry. A parsec is defined as the length of the adjacent to the vertex occupied by a star whose parallax angle is one arcsecond

12.
Delta Canis Majoris
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Delta Canis Majoris, also named Wezen, is a star in the constellation of Canis Major. It is a yellow-white F-type supergiant with an apparent magnitude of +1.83, since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. Delta Canis Majoris is the third-brightest star in the constellation after Sirius and Adhara, with an apparent magnitude of +1.83, lying about 10 degrees south southeast of Sirius, it only rises to about 11 degrees above the horizon at the latitude of the United Kingdom. The open cluster NGC2354 is located only 1.3 degrees east of Delta Canis Majoris, as with the rest of Canis Major, Wezen is most visible in winter skies in the northern hemisphere, and summer skies in the southern. In Bayers Uranometria, it is in the Great Dogs hind quarter, δ Canis Majoris is the stars Bayer designation. The traditional name, Wezen, is derived from the medieval Arabic وزن al-wazn, the name was for one of a pair of stars, the other being Hadar, which has now come to refer to Beta Centauri. It is unclear whether the pair of stars was originally Alpha and Beta Centauri or Alpha, in any case, the name was somehow applied to both Delta Canis Majoris and Beta Columbae. Richard Hinckley Allen muses that the name alludes to the difficulty the star has rising above the horizon, astronomer Jim Kaler has noted the aptness of the traditional name given the stars massive nature. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog, the WGSNs first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included Wezen for this star. In Chinese, 弧矢, meaning Bow and Arrow, refers to an asterism consisting of δ Canis Majoris, η Canis Majoris, HD63032, HD65456, ο Puppis, k Puppis, ε Canis Majoris, κ Canis Majoris and π Puppis. This star, along with ε CMa, η CMa and ο2 CMa, were Al ʽAdhārā, Wezen is a supergiant of class F8 with a radius around 237 times that of the Sun. Its surface temperature is around 5,818 K, and it weighs around 17 solar masses and its absolute magnitude is –6.87, and it lies around 1,600 light-years away. It is rotating at a speed of around 28 km/s, only around 10 million years old, Wezen has stopped fusing hydrogen in its core. Its outer envelope is beginning to expand and cool, and in the next 100,000 years it become a red supergiant as its core fuses heavier and heavier elements. Once it has a core of iron, it will collapse, if Wezen were as close to Earth as Sirius is, it would be as bright as a half-full moon. Wezen appears on the flag of Brazil, symbolising the state of Roraima

A Chandra X-ray Observatory image of the Sirius star system, where the spike-like pattern is due to the support structure for the transmission grating. The bright source is Sirius B. Credit: NASA/SAO/CXC.

An artist's impression of Sirius A and Sirius B. Sirius A is the larger of the two stars.

Hipparcos was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It …

Hipparcos satellite in the Large Solar Simulator, ESTEC, February 1988

Artist's concept of our Milky Way galaxy, showing two prominent spiral arms attached to the ends of a thick central bar. Hipparcos mapped many stars in the solar neighbourhood with great accuracy, though this represents only a small fraction of stars in the galaxy.

Image: Hipparcos insignia

Equirectangular plot of declination vs right ascension of stars brighter than apparent magnitude 5 on the Hipparcos Catalogue, coded by spectral type and apparent magnitude, relative to the modern constellations and the ecliptic

The entire sky, divided into two halves. Right ascension (blue) begins at the vernal equinox (at right, at the intersection of the ecliptic (red) and the equator (green)) and increases eastward (towards the left). The lines of right ascension (blue) from pole to pole divide the sky into 24h, each equivalent to 15°.

In astronomy, declination (abbreviated dec; symbol δ) is one of the two angles that locate a point on the celestial …

The night sky, divided into two halves. Declination (green) begins at the equator (green) and is positive northward (towards the top), negative southward (towards the bottom). The lines of declination (green) divide the sky into small circles, here 15° apart.